Sexual Size Dimorphism and Ecomorphology of Spotfin Shiner (Cyprinella spiloptera) from the Wabash River Watershed

Abstract

We quantified body morphology in individuals from 49 collections of spotfin shiner (Cyprinella spiloptera) in the Wabash River watershed. Ten morphological measurements were used to produce principal component (PC) axes that were examined for relationships with watershed area. Analyses of males and females resulted in sexual dimorphism for one PC axis that represented caudal fin length and head depth; both have increased size in males. An analysis of body shape with watershed area resulted in different relationships for males and females. Caudal peduncle length increased with stream size for males and decreased with stream size for females. No significant relationships were found for sexual size dimorphism (SSD) with stream size. Allometry for SSD supported Rensch's rule -male body size increased more rapidly than female body size with increased SSD. Morphology in spotfin shiners is constrained by a combination of sexual selection and the hydrologic regime of the watershed.

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

“…Here, we complement Dale and coworkers' findings by providing the first experimental evidence that a sexual selection gradient among social groups drives growth and body-size plasticity, and a pattern of positive allometry for SSD. Our study highlights both the usefulness of phenotypic plasticity for testing evolutionary theories (Warner 1991), and the role that individual growth modification can play in the shaping of morphological patterns in nature (see also Buston 2003;Fairbairn 2005;Pyron et al 2007;Kohda et al 2008;Lengkeek et al 2008).…”

“…As poikilotherms, moreover, fishes are extremely sensitive to prevailing physical environmental factors, particularly temperature (Atkinson 1994). Fish are therefore likely to experience growth and body-size trade-offs frequently in relation to conflicting environmental selection pressures (Young 2005;Pyron et al 2007;Lengkeek et al 2008). While it is unlikely that physical factors played a significant role in determining the patterns of SSD reported here (owing the extremely small spatial scale of the study), plasticity studies at larger spatial scales should incorporate physical variables in the analysis to get at the true relationships between ultimate selection pressures and patterns of SSD.…”

Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females

“…Originally, it was thought that interspecifi c allometry for SSD refl ects the genetic response of individual species to selection through evolutionary time (Fairbairn 1997). On the other hand, it is possible that intraspecifi c variation in body size results from phenotypic plasticity, local genetic adaptations (Fairbairn 2005;Pyron et al 2007;Lengkeek et al 2008), or migration to the populations suited for their own phenotypes. In L. callipterus, body size plasticity might be a possible explanation for intraspecifi c allometry for SSD.…”

“…male size increases allometrically with female size across populations (i.e. Δsize male >Δsize female ; Fairbairn and Preziosi 1994;Fairbairn 2005;Young 2005;Blanckenhorn et al 2006;Pyron et al 2007;Lengkeek et al 2008; see also Walker and McCormick 2009). This pattern is known When males are the larger sex, a positive allometric relationship between male and female sizes is often found across populations of a single species (i.e.…”

Effects of Allometry, Sex, and River Location on Morphological Variation of Freshwater Drum Aplodinotus grunniens in the Wabash River, USA

“…Sexual dimorphism in shape differs with sex-specific ontogeny and differences in timing of sexual maturity and investments of energy into egg and sperm production. For example, Pyron et al (2007) identified sex-specific shape responses to stream discharge variation, suggesting a tradeoff between sexual selection and natural selection in a North American cyprinid. However, identification that sexual selection is a cause of observed morphological variation requires additional evidence of an association with intrasexual competition and/or mate preferences.…”